How the R1 replication control system responds to copy number deviations.

The copy number of R1 and of a repA-lacZ gene fusion was increased above normal by coupling to a plasmid which is present at a fivefold higher copy number at 30 degrees C. This carrier plasmid is deficient in replication at 42 degrees C, and it was thus possible after a temperature shift to analyze the response to the increased plasmid concentration of the R1 replication control system. Both the frequency of replication per plasmid molecule and the rate of repA expression per gene copy were reduced under these conditions, and the data strongly suggest that there is an inverse proportionality between the specific rate of plasmid replication viz repA expression and the copy number/gene dosage of the plasmid.